Dual effect of inorganic polymeric phosphate/polyphosphate on osteoblasts and osteoclasts in vitro

Inorganic polymeric phosphate/polyphosphate (polyP) is a natural polymer existing in both pro‐ and eukaryotic systems. In the present study the effect of polyP as well as of polyP supplied in a stoichiometric ratio of 2  m polyP:1  m CaCl 2 [polyP (Ca 2+ complex)] on the osteoblast‐like SaOS‐2 cells and the osteoclast‐like RAW 264.7 cells was determined. Both polymers are non‐toxic for these cells up to a concentration of 100 µ m . In contrast to polyP, polyP (Ca 2+ complex) significantly induced hydroxyapatite formation at a concentration > 10 µ m , as documented by alizarin red S staining and scanning electron microscopic (SEM) inspection. Furthermore, polyP (Ca 2+ complex) triggered in SaOS‐2 cells transcription of BMP2 (bone morphogenetic protein 2), a cytokine involved in maturation of hydroxyapatite‐forming cells. An additional activity of polyP (Ca 2+ complex) is described by showing that this polymer impairs osteoclastogenesis. At concentrations > 10 µ m polyP (Ca 2+ complex) slows down the progression of RAW 264.7 cells to functional osteoclasts, as measured by the expression of TRAP (tartrate‐resistant acid phosphatase). Finally, it is shown that 10–100 µ m polyP (Ca 2+ complex) inhibited phosphorylation of I κ B α by the respective kinase in RAW 264.7 cells. We concluded that polyP (Ca 2+ complex) displays a dual effect on bone metabolizing cells. It promotes hydroxyapatite formation in SaOS‐2 cells (osteoblasts) and impairs maturation of the osteoclast‐related RAW 264.7 cells. Copyright © 2012 John Wiley & Sons, Ltd.